General Two-Phase Routes to Synthesize Colloidal Metal Oxide Nanocrystals: SimpleSynthesis and Ordered Self-Assembly Structures

Thanh-Dinh Nguyen and Trong-On Do*Department of Chemical Engineering, LaVal UniVersity, Quebec G1K 7P4, Canada

ReceiVed: January 9, 2009; ReVised Manuscript ReceiVed: May 1, 2009

Different two-phase approaches have been developed for the synthesis of two classes of monodisperse colloidalmetal oxide nanocrystals (NCs): rare earth oxide NCs and transition metal oxide NCs. These routes weresimple and inexpensive, using metal salts instead of organometallic compounds, with mild reaction conditions,easily controlled size and shape, and multigram-scale products. The obtained products were characterized bytransmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray diffraction (XRD),X-ray photoelectron spectra (XPS), Fourier transform infrared absorption spectroscopy (FTIR), and nitrogenadsorption-desorption isotherms (BET). The possible mechanisms for the formation and growth of nanocrystalswere discussed. Accordingly, tert-butylamine (nucleophile agent) and ethanol (reduced agent) are the keyfactors for the formation of rare earth oxide NCs and transition metal oxide NCs, respectively. Differentsizes and shapes of monodisperse nanoparticles such as spherical, cubic, peanut, rod, and hexagonal NCswere obtained, depending on the nature of metal precursors. Furthermore, the effect of monomeric precursorconcentration, type of precursors, and reaction time on the size and shape of the products was also studied.The SAED patterns of the obtained NC samples show a set of sharp spots that are characteristic of singlecrystalline structures indexed accordingly with the structures determined by the XRD spectra. The XPS resultsrevealed the presence of two oxidation states of cerium, samarium, manganese, and cobalt on the nanocrystalsurface, whereas it seems only one oxidation state is present on the surface for Y, Cr, La, Gd, and Er oxidenanocrystals. A large O 1s XPS peak attributed to two oxygen components for these Ce, Sm, Mn, and Cooxide NC samples refer to the defected structure. Our approaches may be also applicable to synthesize otheruniform metal oxide NCs as well as doped metal oxide NCs and multicomponent NCs.

1. Introduction

Colloidal inorganic nanocrystals (NCs), 1-100 nm in size,are versatile building blocks for constructing diverse super-structures, functional mesocrystals, and new nanodevices, whichcan expect to find useful applications in catalysis, magnetic datastorage, solar cells, lithium ion batteries, medicine, etc.1-4

Assembling the single nanocrystals into ordered lattice structuresthat is “bottom up assembly” is an attractive way for us to designsystems possessing the novel physicochemical properties thatdiffer drastically from their bulk counterparts.5,6 Thus, thedevelopment and innovation of new synthetic strategies for high-quality oxide nanocrystals have been among the key issues inscientific research.7

Numerous methods have been well-documented in severalrecent review articles for the synthesis of a broad range ofnanocrystals, including the hydrolytic and nonhydrolytic sol-gelprocess,8 solvo-hydrothermal treatment,9 and thermal decom-position.10 Size and shape controls for nanocrystals can beobtained via single-phase or two-phase methods. The single-phase method is always used for the synthesis of colloidal oxidenanocrystals.11,12 Furthermore, the two-phase synthetic methodwas first reported by Brust et al.13 for the synthesis of goldnanoparticles and other noble metal nanoparticles such as Ag,Pt, and Au.14 Related progress using the two phase approachwas also extended to the synthesis of metal oxide andsemiconductor nanocrystals.15 Compared to the single-phase

approach, the two-phase strategy can obtain the products withgood crystallinity under relatively mild adopted conditionsbecause the presence of the water phase in bulk solutiongenerally increases the speed of the growth process, which waspreviously reported by Lu et al.16 and recently by our group.17

Particle size and shape can also be controlled because of therelatively slow nucleation process, which was demonstrated byAn et al.18 Many attempts have been made to use the designand synthesis of a variety of NCs and this two-phase approachto explore their properties and potential applications. Forexample, Pan et al.19,20 reported a hydrothermal two-phase routeto synthesize monodisperse TiO2 and ZrO2 NCs from thehydrolysis of titanium(IV) isopropoxide and zirconium(IV)isopropoxide, respectively, in a water-toluene mixture. Kaskelet al.21 reported the synthesis of BaTiO3 nanocrystals usingmixed titanium(IV)-n-butoxide and barium acetate precursors.Furthermore, Adschiri et al.22 modified this reaction system forthe synthesis of CeO2 nanocubes using cerium hydroxideprecursors under supercritical water conditions. The productshape could be controlled by tuning the interaction of organicmolecules with various crystallographic planes of fluorite cubicceria. More recently, our group23 developed a modified two-phase method for the synthesis of alkyl chain-capped metalparticles (e.g., Cu and Au) and metal oxide nanoparticles (CeO2,TiO2, and ZrO2), followed by their cooperative assemblage intounusual hybrid metal/metal oxide NCs mesostructured materials.Indeed, these metal/metal oxide hybrid materials exhibited highsurface areas, narrow pore size distributions, and exceptionalcatalytic properties in the oxidation of CO, even surpassing the

* To whom correspondence should be addressed. E-mail: Trong-On.Do@gch.ulaval.ca.

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10.1021/jp900226m CCC: $40.75 2009 American Chemical SocietyPublished on Web 06/05/2009

performance of commercial noble metal catalysts. In addition,the controlled size and shape of vanadia and samaria NCs werealso developed using inexpensive metal precursors and multi-gram scales in a single run.17,24 Even though the two-phasemethods have been shown to synthesize various metal oxidenanocrystals, the development of general and simple two-phasemethods for the synthesis of a variety of NCs with a desiredshape and size control using inexpensive precursors still faceschallenges.

In this paper, we report two different general two-phaseapproaches for the synthesis of two classes of colloidal metaloxide nanocrystals (NCs). These are the rare earth oxide NCsand transition metal oxide NCs, with controlled size and shape,through the hydrolysis of corresponding inorganic salts ormetal-surfactant complexes in the water-toluene system. Thesesynthetic approaches have several advantages, including rela-tively mild conditions, easy manipulation, and large-scaleproducts. The possible mechanisms for the formation of alkyl-capped oxide nanocrystals in each route are also proposed.Furthermore, the effect of various synthetic reaction parameters,including precursor concentration, type of precursors, andreaction time on the shape evolution of nanocrystalline oxidesare also studied.

2. Experimental Section

2.1. Starting Materials. All chemicals were used as re-ceived without further purification. Cerium(III) nitrate hexahy-drate (Ce(NO3)3 ·6H2O, 99.9%), cerium(III) acetate hydrate[Ce(ac)3 ·xH2O, 99.9%], samarium(III) nitrate hexahydrate[Sm(NO3)3 ·6H2O, 99.9%], yttrium(III) nitrate hexahydrate [Y-(NO3)3 ·6H2O, 99.9%], lanthanium (III) nitrate hexahydrate [La-(NO3)3 ·6H2O, 99.9%], gadolinium(III) nitrate hexahydrate[Gd(NO3)3 ·6H2O, 99.9%], erbium(III) nitrate hydrate [Er(NO3)3 ·xH2O, 99.9%], manganese(III) nitrate tetrahydrate [Mn(NO3)2 ·4H2O, 99%], chromium(III) chloride hexahydrate (CrCl3 ·6H2O,99%), cobalt(II) nitrate hexahydrate [Co(NO3)3 ·6H2O, 98%],nickel(II) nitrate hexahydrate [Ni(NO3)2 ·6H2O, 99.999%], oleicacid (C18H35COOH or OA, technical grade, 90%), oleylamine(C18H35NH2 or OM, technical grade, 70%), potassium oleate(C18H35COOK, 40% water), and tert-butylamine [(CH3)3CNH2,98%] were purchased from Sigma-Aldrich. All solvents used suchas toluene and absolute ethanol were of analytical grade andpurchased from Reagent ACS.

2.2. Nanocrystal Synthesis. All NCs in this work weresynthesized in a two-phase system and stabilized with hydro-phobic molecules containing long alkyl chains (C18). Detailedinformation on the synthesis of metal oxide NCs is given below.

2.2.1. SynthesisofRareEarthOxideNanocrystals.2.2.1.1. Syn-thesis of Rare Earth Oxide Nanocrystals (e.g., Sm2O3 and CeO2

NCs) Using Rare Earth Salt Precursors. The synthetic proce-dures were similar to that of Sm2O3 NCs, which was recentlyreported by our group.24 Typically, 20 mL of an aqueous solution(0.015-0.120 mol/L) containing a metal salt [0.3-2.4 mmol,Sm(NO3)3 ·6H2O, Ce(NO3)3 ·6H2O, or Ce(ac)3], and tert-buty-lamine (0.15 mL) was mixed with an organic solution of oleicacid (1.5 mL) and toluene (20 mL). The resulting mixture wastransferred to a 60 mL Teflon-lined stainless steel autoclave inthe ambient environment without stirring. The sealed autoclavewas heated at 180 °C for 24 h. Afterward, the system was cooledto room temperature by submerging the reactor in a water bath.The nanocrystal solution was precipitated with the excessvolume of ethanol. The purified products were separated bycentrifugation and easily redispersed in a nonpolar solvent (e.g.,toluene, hexane, and chloroform). The precipitation-redispersion

process was repeated several times to purify the producingnanocrystals.

2.2.1.2. Synthesis of Rare Earth Oxide Nanocrystals UsingRare Earth-Surfactant Complex Precursors (e.g., RE ) Er,Gd, La, or Y). The synthesis of rare earth (RE) oxide nano-crystals consists of two steps: (i) preparation of rare earthcomplex precursors from rare earth salts and (ii) the formationof NCs.

(i) To prepare rare earth-oleate complexes, we produced anorganic solution by adding 20 mL of toluene into the ethanolsolution (6.4 mL) containing potassium oleate (0.85 g or 1.1mmol). Subsequently, the organic phase was mixed with 12.8mL of an aqueous solution of RE(NO3)3 ·6H2O (0.36 mmol,RE ) Er, Gd, La, or Y) and transferred to a flask. The stocktwo-phase mixture was heated to 70 °C for 60 min with stirringvigorously, and the organic solution turned light yellow afterthe reaction, indicating the occurrence of the coordinativereaction between a rare earth cation and oleate anion for thecomplex formation. The RE3+/oleate- molar ratio is close to1:3, RE(OA)3. The upper homogeneous toluene supernatantphase (20 mL) containing RE-oleate complexes was isolatedusing a separatory funnel. The yields for these rare earthcomplexes were between 85% and 95%.

(ii) To form Er2O3, Gd2O3, La2O3, and Y2O3 nanocrystals,we typically added 5 mL of oleylamine to the above-preparedrare earth complex solution (20 mL, 0.018 mol/L) under stirringfor 10 min. Then, the organic solution was transferred to a 60mL Teflon-lined stainless steel autoclave containing an aqueoussolution (20 mL) of tert-butylamine (0.15 mL). The autoclavewas seated and heated to the crystallization temperature at 180°C for 24 h.

2.2.2. Synthesis of Transition Metal Oxide Nanocrystals(TMO-NCs) (e.g., Mn3O4, Cr2O3, Co3O4, or NiO NCs UsingTransition Metal Salts as Precursors. Typically, a homoge-neous ethanol solution (10 mL) containing potassium oleate (3g) was added to the organic solution of toluene (20 mL) andoleic acid (2 mL) and then was mixed with 20 mL of an aqueoussolution (0.08 M, 1.6 mmol) containing a metal salt, includingMn(NO3)3, CrCl3, Co(NO3)2, and Ni(NO3)2, for the formationof a two-phase reactive mixture, in which the oleate anionsdisperse into the organic phase, while the ethanol and potassium(K+) cations enter into the aqueous phase. The two-phasemixture was transferred to a Teflon-lined stainless steel autoclaveand heated to 180 °C for 24 h. The yield of the products wasmore than 85%.

2.3. Characterization. The particle sizes and shapes of thesynthesized oxide nanocrystals were determined at 120 kV bya JEOL JEM 1230 transmission electron microscope (TEM).Samples were prepared by placing a drop of a dilute toluenedispersion of nanocrystals onto a 200 mesh carbon-coated coppergrid and immediately evaporated at ambient temperature. Theaverage particle dimensions were determined by size distributiondiagrams, which were obtained from about 100-150 particlesin representative TEM pictures of each sample.

The nanocrystal products were characterized on a BrukerSMART APEX II X-ray diffractometer and operated at 1200W power (40 kV, 30 mA) to generate Cu KR radiation (λ )1.5418 Å). The X-ray photoelectron spectra (XPS) were takenon a photoelectron spectrometer (Kratos Axis-Ultra) with amonochromatic X-ray source of Al KR. The positions of theXPS peaks were corrected using the C 1s peak at 285 eV as areference. The peaks were deconvoluted by means of a standardCasaXPS software (v.2.3.13; product of CasaXPS Software Ltd.,U.S.A.) in order to resolve the separate constituents after

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background subtraction. Fourier transform infrared absorption(FTIR) spectra were measured with a FTS 45 infrared spectro-photometer using the KBr pellet technique. The specific areawas calculated from the linear part of the Brunauer-Emmett-Teller (BET) equation (P/P0 ≈ 0.05-0.20). The pore sizedistribution was obtained from the analysis of the desorptionbranch of the isotherms using the Barrett-Joyner-Halendamodel.

3. Results and Discussion

All of the syntheses in this study were carried out in a two-phase water-toluene system by mixing metal sources andcapping agents. After hydrothermal synthesis, the cappednanocrystal products were highly dispersed in the toluene phase,and no product was observed in the water phase. The NCproduct in the toluene phase was precipitated by adding excessof ethanol and recovered by centrifugation. Table 1 summarizesthe synthesis conditions, the metal precursors, and cappingligands for the synthesis of nanocrystals (NCs) as well as thesize and shape of the as-made samples determined by transmis-sion electron microscopy (TEM). Furthermore, even aftercalcination, no significant change in the morphology of thesesamples was observed (S-Figure 9 of the Supporting Informa-tion). This could be due to the particle surface protected bycapping agents during thermal treatment. In this study, we havedeveloped two different simple and general two-phase routesfor the synthesis of metal oxide NCs of two-group elements:rare earths, Sm, Ce, Er, Gd, La, and Y (Schemes 1A,B) andtransition metals, Mn, Cr, Co, and Ni (Scheme 2), using metalsalts as starting precursors. There are some advantageous forthese routes because of the use of inorganic salt precursors,instead of expensive metal alkoxides, and quite mild syntheticconditions. Particularly, these synthesis methods are scalableto multigram using the same conditions used in the milligramones as shown in S-Figure 8 of the Supporting Information.Two to three grams of the product can be obtained in a singlepreparation. Furthermore, the shape and size of the NC productscan also be controlled by various reaction parameters, includingthe monomer concentration of metal precursors, type of precur-sors, and reaction time.

3.1. Synthesis of Rare Earth Oxide Nanocrystals. To date,a number of rare earth oxide nanocrystals are being studiedintensively because of their diverse applications due to their

unique 4f electrons,25 including electronic, magnetic, optical,and catalytic fields. Many rare earth nanostructures have beensynthesized through solution-based routes.26 Among them, thehydrothermal two-phase route is particularly versatile for thesynthesis of diverse rare earth oxide nanocrytals. However, thisapproach is rather restricted because metal alkoxides as precur-sors are commercially not available or expensive.

To overcome these drawbacks, in this work, we present twoalternative two-phase routes to synthesize nanocrystalline rare

TABLE 1: Synthesis Conditions and Morphologies of the As-Made Capped Metal Oxide Nanocrystals via HydrothermalTwo-Phase Routes

samplea precursorprecursorconcn (M)

aqueousphase (mL) organic phase surfactant (mL) time (h) product morphology

rare earth oxide nanocrystals (Schemes 1A,B)1 Sm(NO3)3 0.120 water, TBAb toluene OAb 24 Sm2O3 rod2 Ce(NO3)3 0.120 water, TBA toluene OA 24 CeO2 cubic, elongated cubic self-assembly3 Ce(ac)3 0.120 water, TBA toluene OA 24 CeO2 agglomerated cubic self-assembly4 Er-oleate 0.018 water, TBA toluene OMb 24 Er2O3 spherical self-assembly5 Gd-oleate 0.018 water, TBA toluene OM 24 Gd2O3 spherical,peanut6 La-oleate 0.018 water, TBA toluene OM 24 La2O3 short rod7 Y-oleate 0.018 water, TBA toluene OM 24 Y2O3 long rod

transition metal oxide nanocrystals (Scheme 2)8 Mn(NO3)2 0.08 water, ethanol toluene, KOAb OA 24 Mn3O4 hexagonal self-assembly9 CrCl3 0.08 water, ethanol toluene, KOA OA 24 Cr2O3 dot

10 Co(NO3)2 0.08 water, ethanol toluene, KOA OA 24 Co3O4 spherical self-assembly11 Co(NO3)2 0.08 water, ethanol toluene, KOA OA 18 Co3O4 cubic12 Co(NO3)2 0.08 water, ethanol toluene, KOA OA 12 Co3O4 cubic,assembled cubic13 Ni(NO3)2 0.08 water, ethanol toluene, KOA OA 24 NiO hexagonal

a All samples were synthesized by solvothermal processes at 180 °C. b TBA (tert-butylamine), KOA (potassium oleate), OA (oleic acid), andOM (oleylamine).

SCHEME 1: (A) Two-Phase Route to Synthesize RareEarth Oxide Nanocrystals Using Rare Earth SaltPrecursors and (B) Two-Phase Route to Synthesize RareEarth Oxide Nanocrystals Using Rare Earth-OleateComplex Precursors

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earth oxides in a water-toluene mixture in the presence ofcapping agents using (i) direct rare earth nitrate salts (Scheme1A) and (ii) the rare earth-surfactant complexes prepared fromthe corresponding nitrate salts (Scheme 1B) as starting precursors.

3.1.1. Synthesis of Earth Oxide NCs Using Rare Earth SaltPrecursors. Nanocrystalline rare earth oxides were obtained bythe hydrolyzed reaction of the corresponding nitrate salts in awater-toluene mixture in the presence of capping molecules(fatty acid) and tert-butylamine (Experimental Section). Thissynthetic route shows the distinct advantage of relatively lowtemperature compared with the supercritical water method inthe absence of tert-butylamine as an activation agent as reportedby Adschiri et al.22 Sm2O3 and CeO2 were selected to illustratethis approach because they are widely recognized for theirunique properties as supports and catalysts. The formation ofrare earth oxide NCs is sketched in Scheme 1A.

In this way, the formation of Sm2O3 and CeO2 NCs can beexpressed in Scheme 1A. It is well-known that the Ce4+ ionshave stable empty 4f0 subshells;27 therefore, the Ce3+ ions wereeasily oxidized to the Ce4+ ions by the trace oxygen existing inthe aqueous solution, according to the suggestion of Xing elal.28 The metal hydroxides were formed under basic solutiondue to the hydrolysis of tert-butylamine in water to generateOH- ions.29 Upon hydrothermal treatment, these metal hydrox-ides were dehydrated to oxide nuclei at the interfaces.

Figure 1 shows the TEM images of Sm2O3 and CeO2 NCs(samples 1 and 2 in Table 1) prepared at a high metal monomerconcentration of the corresponding nitrate salts in the watersolution (e.g., C ) 0.12 M) following Scheme 1A. Under thesesynthesis conditions, single-crystalline Sm2O3 nanorods, ∼7 nmin width and 100-160 nm in length (aspect ratio of ∼19), wereobserved (Figure 1a); meanwhile, CeO2 NCs composed of amixture of nanocubes (6 nm) and elongated nanocubes (8 nm× 10 nm) were obtained. CeO2 NCs had a tendency to assembleinto 2D arrays (Figure 1c). The selected area electron diffraction(SAED) patterns taken by focusing the electronic beam on singlesamaria and ceria particles are presented in Figure 1b,d,respectively, showing the distinct diffraction rings of cubicstructures, which is in good agreement with the XRD results(see below).

The size and shape evolution of CeO2 NCs was found to bea function of cerium monomer concentrations, e.g., Ce(NO3)3,in the water solution. Representative TEM images of the samplesare shown in Figure 2. At a relatively low cerium monomerconcentration in the water phase (C ) 0.015 M), only orderedCeO2 nanocubes were formed with an average diameter of 3nm (Figure 2a). However, their cube size increased from 3 to8 nm when the cerium precursor concentration in the initialwater phase was 0.068 M (Figure 2b). As observed under careful

high-magnification TEM examination (inset of Figure 2b), themonodisperse nanocubes had strong tendency to assemble intomultilayer superlattices (two-dimensional). By increasing themonomer concentration to 0.120 M, it yields some elongatednanocubes (8 nm × 10 nm) with sharper edges and corners.The self-assembly of the mixture of cubes and elongated cubesNCs was observed (Figure 2c). The general trend is alsosketched in Figure 2d. The elongation and size of ceriananocubes increased as the concentration of cerium monomersin the initial water solution increased. It is clear that theconcentration of cerium salts plays an important role for thesizes and shapes of the resulting nanocubes. On the basis ofthe Gibbs-Thompson theory,30 the chemical potential of acrystal is proportional to its surface-atom ratio; however, thesize and shape evolution of NCs is strongly dependent on metalmonomer concentrations as reported by Peng’s group.31,32

Generally, low monomer concentrations favor isotropic growth,whereas high monomer concentrations favor anisotropic growth.

Furthermore, the effect of the nature of cerium precursorson the size and shape of NCs was also studied. When ceriumnitrate was replaced by cerium acetate (C ) 0.068 M, acetateis weaker anion than nitrate33,34), some aggregated particlescomposed of two or three of the individual nanocubes withrelatively larger sizes (8-10 nm) were observed (Figures 2eand 2f). This suggests that the nature of the counteranion ligandsplayed a role in the growth of uniform nanocubes. Li et al.33

also found that the variation of type of anion in the water phasecould change the growth rate and, consequently, change the sizeand shape of Cu2S nanocrystals.

Figure 3 shows the XRD patterns of the various calcined NCsamples: Sm2O3 nanorods, CeO2 nanocubes, and agglomeratedCeO2 nanocubes (samples 1, 2, and 3 in Table 1). The resultsrevealed the body-centered cubic (bcc) structure of these sam-ples with cell parameters for Sm2O3 of a ) 10.93 Å (Ia3, JCPDS86-2479) and for CeO2 of a ) 5.45 Å (Fm3m, JCPDS34-0394). As seen in Figure 3, the XRD patterns show broadpeaks due to the nanoscale size of the products, and no peaksof other impurities were detected. The sizes of these samples,

SCHEME 2: Two-Phase Route to Synthesize TransitionMetal Oxide Nanocrystals Using Metal Salt Precursors

Figure 1. Rare earth oxide nanocrystals synthesized using correspond-ing nitrate salts at a high monomer concentration (C ) 0.120 M)following Scheme 1A. (a) TEM image and (b) SAED pattern of 7 nm× 100-160 nm-sized Sm2O3 nanorods (sample 1). (c) TEM imageand (d) SAED pattern of the CeO2 NC mixture of 8 nm-sized nanocubesand 8 nm × 10 nm-sized elongated nanocubes (sample 2).

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corresponding to samples 2 and 3 in Table 1 and calculatedfrom the half-widths of the (220) reflections35 using theDebye-Scherrer equation (d ) 0.89λ/Bcos θ), are ∼11.0 and∼10.5 nm, respectively, which are consistent with the TEMresults (S-Figure 9 of the Supporting Information).

The XPS spectra of metal oxides reveal that the metal corelevel binding energies and full widths at half-maximum (fwhm)taken from nanoparticles are obviously higher than those fromthe counterpart bulk.36 The upward shift of binding energies byabout 0.5-1.5 eV might result from the dispersion of metalion clusters on the particle surfaces, while the increasing fwhmof about 2.5-3.5 eV might cause a change in electronicproperties with decreasing particle size.37

The obtained cerium oxide NC products before and aftercalcination have a light yellow color, but pure bulk CeO2

powders have a white color, indicating the chemical compositionof cerium oxide NCs may be CeO2-x (0 < x < 2). Replacementof the smaller Ce4+ ions (∼0.92 Å radii) by a few larger Ce3+

ions (∼1.034 Å radii) into the crystal lattice may result in thecolor change from white to light yellow. To further confirmthis observation, we recorded the Ce 3d core level XPS spectra(Figure 4) for the sample of ceria nanocubes (solid curve) andafter calcination (dotted curve, sample 2 in Table 1). The XPSspectra in wide energy ranges are also presented in S-Figure 1

of the Supporting Information; beside carbons, no impuritieswere found in this sample. It is interesting to note that nosignificant difference in the XPS spectrum before and aftercalcination of these samples was observed, suggesting nosignificant change in the cerium oxidation state with this thermaltreatment. The Ce 3d states are split due to spin-orbit interactioninto two lines: Ce 3d5/2/Ce 3d3/2 with a spin energy separation(∆) of 18.3 eV and an intensity of I(3d5/2)/I(3d3/2) ) 1.1.38 Thecomplex spectrum of Ce 3d can be decomposed to sixcomponents with the assignment as defined in Figure 4. Thebands at 887.50 eV (V′′) and 903.76 eV (u′′) are the 3d104f1

initial electronic state of the Ce3+ ion, and the bands at 880.30eV (V′), 887.50 eV (V′′), 887.50 eV (V′′′), 898.64 eV (u), 903.76eV (u′′), and 916.35 eV (u′′′) are the 3d104f0 state of the Ce4+

ion.37 The atomic ratio of Ce3+/(Ce3+ + Ce4+) of the as-madeand calcined ceria NC samples can be quasi-quantitativelydetermined by the ratio of (Iv′′+ Iu′′)/(Iu + Iv + 2Iv′′ + 2Iu′′ +Iv′′′ + Iu′′′), which is ∼46%. Furthermore, the Ce/O ratio wasfound to be 1.00:1.67, corresponding the stoichiometric formulaof CeO2-x (x ) 0.33). This suggests that the distorted crystalstructure of nanoceria due to the replacement of Ce4+ ions byCe3+ ions causes an oxygen vacancy creation in surface defects.The presence of two oxidation states (Sm2+/Sm3+) with 40%Sm2+ and the chemical composition of Sm2O(3-1.8) on thesamaria NC surface were observed in our recent study.24

Figure 2. Shape and size evolution of CeO2 nanocrystals as a functionof cerium monomer concentrations (C). TEM images of CeO2 NCsamples prepared at (a) C ) 0.015 M, 3 nm-sized nanocubes, (b) C )0.068 M, 8 nm-sized nanocubes, and (c) C ) 0.120 M, a mixture of 8nm-sized nanocubes and 8 nm × 10 nm-sized elongated nanocubes.(d) Correlation plot showing the relationship of the cerium monomerconcentration in a water phase and average diameter of CeO2 nanocubes.(e,f) 8-10 nm-sized agglomerated CeO2 nanocubes obtained usingCe(ac)3 precursors (C ) 0.068 M, sample 3).

Figure 3. XRD patterns of the calcined rare earth oxide NC samples: (a)Sm2 (sample 1), (b) CeO2 NC mixture of nanocubes and elongatednanocubes (sample 2), and (c) agglomerated CeO2 nanocubes (sample 3).

Figure 4. Ce 3d XPS spectra of the nanoceria (sample 2) before (solidcurve) and after (dotted curve) calcination.

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3.1.2. Synthesis of Rare Earth Oxide NCs Using RareEarth-Surfactant Complex Precursors. Recently, we havereported the shape selective synthesis of vanadium oxidenanocrystals (NCs) through the decomposing of vanadium-ligand complexes in the water-toluene mixture in the presenceof aliphatic amine.17 It was found that the size and shape ofNCs can be controlled by various synthesis parameters such aswater content, steric ligands of vanadium complexes, and thealkyl chain length of capping agents. Some authors haddeveloped the two-phase method to produce the semiconductornanocrystals.39 However, few reports described a general routefor rare earth oxide nanocrystal synthesis as well as studies ofthe reaction mechanism. From these points of view, we extendinto a general method and study of the reaction mechanism forthe synthesis of different types of rare earth oxide nanocrystals(REO-NCs) using rare earth-oleate complexes. In this work,Er, Gd, La, and Y oxide NCs were selected and synthesizedwith controllable size and shape because of their wide varietyof technological applications and limited information availablein the literature for the synthesis of these NCs through two-phase approach.18

The synthesis of REO-NCs involves two steps (Scheme 1B):(i) the preparation of rare earth-oleate complexes from thereaction between rare earth nitrate and potassium oleate (KOA)in a water-toluene system and (ii) the formation of colloidalREO-NCs in the autoclave containing the water-toluenemixture composed of capping oleylamine, tert-butylamine, andrare earth-oleate complexes at 180 °C. All of the rareearth-oleate complexes (Er, Gd, La, and Y) were synthesizedfrom the corresponding rare earth nitrate compounds using thesame procedure. For example, the FTIR spectrum of theY-oleate complex precursor (after toluene elimination) is shownin S-Figure 2 of the Supporting Information. The absorptionband at 731 cm-1 is assigned to the stretching vibrations of theY-O bonds.40 Two absorption bands are observed at 1466 cm-1

and 1548 cm-1, which are attributed to the symmetric andasymmetric stretching vibrations of the carboxylate groups ofoleate ligands, respectively. The strong absorption bands at2849-2928 cm-1 are characteristic of the symmetric andasymmetric methyl and methylene stretches of the alkyl chains.These FTIR bands are characteristic of Y-oleate complexes.41

The rare earth nanocrystals (RE-NCs) synthesized withdifferent morphologies are summarized in Table 1. To proveas-made RE-NCs are protected by oleylamine molecules, arepresentative FTIR spectrum of the OM-capped Y2O3 sampleis presented in S-Figure 3a of the Supporting Information. Itclearly indicates oleylamine molecules bonded on the nano-crystal surface. Figure 5 shows the XRD patterns of varioustypes of calcined NC products. The Er2O3, Gd2O3, and Y2O3

samples exhibit cubic structures, while the La2O3 sample showsa hexagonal structure. The calculated lattice constants are 10.63,11.28, 10.80, 10.56, and 3.39 Å for Y2O3 (JCPDS 25-1200),41

Gd2O3 (JCPDS 86-2477),42 Er2O3 (JCPDS 08-0050),43 and La2O3

(JCPDS 05-0602), respectively.44 The particle sizes of Gd2O3

and Er2O3 NCs calculated from the (222) reflections are about20 and 30 nm, respectively, which are consistent with the sizesobtained by the TEM images (see below and S-Figure 9 of theSupporting Information). The XRD spectra of all of thenanocrystal samples show much broader diffraction peakscompared to those of their bulk counterparts because of theirnanometer size.

It should be noted that tert-butylamine and rare earth-oleatecomplexes play a key role for the formation of rare earthnanocrystals. For example, the synthesis of Y2O3 NCs was

carried out under the same synthesis conditions, and except forthe absence of tert-butylamine in the synthesis mixture, no rareearth NC product was formed. When yttrium acetate compoundswere used instead yttrium-oleate complexes, keeping the othersynthesis conditions constant (e.g., in the presence of cappingoleylamine and tert-butylamine), only irregular Y2O3 nano-spheres were obtained (S-Figure 4 of the Supporting Informa-tion).

The formation of NCs using rare earth-oleate complexes,capping oleylamine, and tert-butylamine could be expressed ineq 1. The formation of RE-NCs could consist of three stepsduring the hydrothermal synthesis: (i) Tert-butylamine asnucleophile attacks one carboxyl group of the oleate ligandthrough the sharing of a lone pair of electrons of the donor NH2

group with the electrophilic carboxyl center at the nucleationstage. Tert-butylamine can react with the oleate group but notwith oleylamine because its basic property is stronger than thatof oleylamine. Moreover, tert-butylamine is dispersed anaqueous phase and easily hydrolyzed to generate OH- comparedwith oleylamine in organic phase. (ii) This nucleophilic reactionleads to C-O bond cleavage, the release of a carbonyl group,and then the formation of REO nuclei at the water-tolueneinterfaces. (iii) The formation of NCs. A similar observationwas also reported for the synthesis of ZnO nanocrystals in themixture of zinc acetate and oleylamine.45

Figures 6-8 show representative TEM images and SAEDpatterns of different rare earth oxide NCs (Table 1). Under thesame synthesis conditions, different shapes of the final NCproducts were observed: spheres for Er2O3 (Figure 6), themixture of spheres and peanuts for Gd2O3 (Figure 7), and rodsfor La2O3 and Y2O3 (Figure 8), depending on metal-oleatecomplexes. Er2O3 nanospheres with a size of about 10-20 nmwere obtained, meaning that no growth direction is dominantduring the seeded growth, whereas nanorods were yielded for

Figure 5. XRD patterns of calcined rare earth oxide NC samplessynthesized using corresponding rare earth-oleate complexes followingScheme 1B: (a) Er2O3 nanospheres (sample 4), (b) Y2O3 nanorods(sample 7), (c) La2O3 nanorods (sample 6), and (d) Gd2O3 nanospheres/nanopeanuts (sample 5).

J. Phys. Chem. C, Vol. 113, No. 26, 2009 11209

La2O3 and Y2O3. The La2O3 nanorods of 8 nm in width and 20nm in length with an aspect ratio of ∼2.5 were observed, whileY2O3 nanorods (10 nm × 80-120 nm) with the aspect ratio of∼10 were observed. However, for Gd2O3, the mixture of nearlyround-shaped nanocrystals (10-20 nm) and partly nanopeanuts(10-20 nm × 20-40 nm) was identified. The correspondingSAED patterns of these Er2O3, Gd2O3, and Y2O3 NC samplesexhibit a set of sharp spots that are characteristic of singlecrystalline cubic structures, while the diffraction rings of theLa2O3 NC sample corresponds to a hexagonal structure, whichis in good agreement with the XRD results.41-44 It seems thatunit cell structure and crystallographic surface energy of seedsare critical for directing the intrinsic shapes of rare earth oxideNCs (Figure 8d).

Survey XPS spectra were recorded for these samples. TheXPS spectra of the calcined Y2O3 nanorods are shown inS-Figures 5 of the Supporting Information. As seen in Figure5A, no significant presence of impurities was observed, exceptfor the carbon. The XPS spectrum of Y 3d (S-Figure 5B of theSupporting Information) shows only the Y 3d5/2 and Y 3d3/2

peaks centered at 151.55 and 153.47 eV, respectively, whichare attributed to Y3+.46 It suggests only one yttrium oxidationstate in the yttrium oxide NC sample after calcination. Theatomic ratio of Y:O determined from the surface chemical

composition of this sample was 1.05:1.51, corresponding toY2O3. The surface chemical compositions for other calcined NCsamples are found to be Er2O3, Gd2O3, and La2O3 (not shown).

The mesoporous structure of Er2O3 nanospheres and Gd2O3

nanospheres/nanopeanuts after calcination at 550 °C for 2 h wereexamined by N2 sorption measurements (S-Figure 6 of theSupporting Information). The isotherm of both samples is typeIV with an H1 hysteresis loop.24,47 These calcined Er2O3 andGd2O3 samples have a BET surface area of 55 and 35 m2/g,respectively, and average pore diameters of 28 and 18 nm,respectively. The pore size distribution of the Gd2O3 sample(inset of S-Figure 6B of the Supporting Information) shows abimodal pore structure. The bimodal pore size distribution maydue to the slit-shaped pores of the mixture of nanospheres andnanopeanuts.

3.2. Synthesis of Transition Metal Oxide Nanocrystals(TMO-NCs). Transition metal oxides have been widely usedin many applications, especially for catalysis and fuel celltechnology. In addition, size and shape of these materials providea sensitive knob for tuning their properties.48 A number ofchemical routes have been developed to produce nanocrystalsof transition metal oxides with defined and controllable shapes.49

In most cases, nanocrystals were formed at high temperatures.Therefore, the development of approaches for the synthesis of

Figure 6. TEM images and corresponding SAED pattern of Er2

synthesized using Er-oleate complexes.Figure 7. TEM images and corresponding SAED pattern of Gd2O3

nanospheres/nanopeanuts synthesized using Gd-oleate complexes.

11210 J. Phys. Chem. C, Vol. 113, No. 26, 2009 Nguyen and Do

such nanocrystals under mild reaction conditions with size andshape control remains a challenge. We developed a newprocedure for the synthesis of a large variety of monodispersetransition metal oxide nanocrystals through a hydrothermal two-phase approach at relatively low temperature (150-180 °C)using inexpensive compounds such as transition metal salts andpotassium oleate, instead of expensive organometallic com-pounds. The synthetic procedure is illustrated in Scheme 2. Thebiphasic mixture was composed of an aqueous phase of metalion (e.g., metal nitrate or chloride), ethanol, and water and atoluene phase of potassium oleate and oleic acid. Themetal-oleate complex was directly formed in a water-toluenemixture from an ion exchange reaction of a metal cation withan oleate anion in an early mass transfer stage: Mn+ + nRCOO-

f M(COOR)n.50 Under hydrothermal synthesis conditions, theresulting metal-oleate complexes were reduced by ethanol togenerate tiny oxide nuclei at the interfaces and then form metaloxide nanocrystals, which were capped by fatty acid in anorganic phase; no NC product was observed in an aqueousphase. To describe this approach, we selected four differenttransition-metal oxides (Mn3O4, Cr2O3, Co3O4, and NiO) andto synthesize in this study (Table 1). FTIR measurements werecarried out for of these as-made OA-capped TMO-NCs. Arepresentative FTIR spectrum of the as-made OA-capped Co3O4

NC sample (sample 10) is shown in S-Figure 7b of theSupporting Information. The presence of the FTIR bandscorrespond to the stretching frequency of the carboxylate groupsfrom the oleic acid, indicating that these groups were bondedto the NC surface. As seen in Figure 9, the XRD patterns ofthese samples are well-matched to tetragonal Mn3O4 (JCPDS16-0154) with a ) 5.759 Å,51 rhombohedral Cr2O3 (JCPDS 05-0667) with a ) 4.958 Å;52 cubic Co3O4 (JCPDS 48-1719) witha ) 8.084 Å,53 and cubic NiO (JCPDS 04-0835) with a ) 4.193Å.54 A broadening of XRD peaks indicates the nanocrystallinenature of these samples. Furthermore, no other phases weredetected in the spectra, suggesting the pure structure of theproducts. The particle sizes calculated using the Scherrerequation from the (103), (110), (331), and (200) reflections forMn3O4, Cr2O3, Co3O4, and NiO NCs, respectively, were about

12, 7, 5, and 7 nm, which are consistent with the TEM results(S-Figure 9 of the Supporting Information).

Representative TEM images and SAED patterns of thesesamples are shown in Figure 10. The Mn3O4 and NiO NCsexhibit a hexagonal-like shape with an average diameter ofnearly 10 and 8 nm, respectively. Closer TEM observation ofthe Mn3O4 NC sample shows single hexagon self-assembledtwo-dimensional (2D) networks. The Cr2O3 NC sample showsself-assembled nanodots with an average diameter of about 3nm. Furthermore, 6 nm-sized nanospheres and a few truncatednanospheres of Co3O4 prefer to construct the ordered hexagonalself-assembly, as proposed by the simulation in the inset ofFigure 10e. The corresponding SAED patterns of these Mn3O4,NiO, Cr2O3, and Co3O4 NC samples exhibit strong diffractionrings and can be well-indexed to tetragonal, cubic, rhombohe-dral, and cubic structures, respectively.

The self-assembly of Co3O4 nanocubes into nanospheres wasalso observed (Figure 11) as a function of reaction time whenall of the synthesis conditions were kept constant (e.g., cobaltnitrate, solvent volume, potassium oleate, oleic acid, and reactiontemperature corresponding to 0.08 M, 40 mL, 3 g, 2 mL, and180 °C, respectively). During the crystal growth process, thecapping surfactant plays an important role in the size and shapeof the nanoparticles. In this study, oleic acid was used as acapping agent, and its concentration was fixed in the reactionsolution. After heating at 180 °C for a relatively short time,12 h, the TEM image of this sample shows two populations ofparticles sizes (Figure 11a): polydisperse small nanocubes (2-4nm) and relatively monodisperse nanocubes self-assembly (∼5nm). However, after 18 h of synthesis, relatively uniformnanocrystals with an average size of 6 nm were observed (Figure11b). Furthermore, when the synthesis time was extended to24 h, the morphology of the product was remarkably changed.The corners and tips of cubes gradually became smooth, andmonodisperse spherial nanocrystals were formed (Figure 11c).These results indicate that a transformation of the nanocubesto monodisperse 6 nm-sized nanospherical self-assembly occurs

Figure 8. TEM images and corresponding SAED patterns of the rareearth oxide nanocrystals synthesized using the corresponding rareearth-oleate complexes: (a,b) La2O3 nanorods and (c) Y2O3 nanorods.(d) Growth diagram of RE2O3 NCs (RE ) Er/Y).

Figure 9. XRD patterns of the calcined transition metal oxide NCsamples: (a) hexagonal Mn3O4 nanocrystals (sample 8), (b) Cr2O3

nanodots (sample 9), (c) Co3O4 nanospheres (sample 10), and (d)hexagonal NiO nanocrystals (sample 13).

J. Phys. Chem. C, Vol. 113, No. 26, 2009 11211

with an extended reaction time, as suggested in Figure 11d. Themost commonly used model for shape control is the Gibbs-Curie-Wulff facet theorem,55,56 which suggests that the shapeof a crystal is determined by the minimum surface energy ofeach facet of the crystal. This means that crystal growth shouldoccur rapidly at high free energy facets. In the early stages,Co3O4 nanocubes can be formed in anisotropic growth. Nanocubesenclosed by six crystal faces of {100}, {010}, and {001} wereformed by the specific growth of {111} facets of the cubocta-hedral clusters. This also indicates that the {111} surface has ahigher surface energy compared to the {100}, {010}, and {001}surfaces.57,58 When extending the reaction time, the monomerCo(II)-oleate complex concentration was gradually depletedby the nucleation and growth of the nanocrystals. If the reactiontime is sufficiently long (in this case, 24 h), the monomerconcentration should drop to a level that is a lower than thatrequired for a given cubic shape. The cubic shape shouldeventually evolve into the spherical shape, which is most stable

shape, by moving monomers from {111} to {100} faces inintraparticles due to the differences in chemical potentialbetween different facets of the nanocrystals. In this case, a fewtruncated nanospheres were still observed. This may be due tothe insufficient ripening reaction time for the transformation ofcubes to spheres.

These NC samples, manganese oxide, cobalt oxide, andchromium oxide nanocrystals before and after calcination, werecharacterized by the XPS technique (Figure 12 and S-Figure 7of the Supporting Information). The XPS spectra before (solidcurve) and after (dotted curve) calcination of these samples arealmost indentical, suggesting no significant change in theoxidation state upon this thermal treatment (Figure 12). Thedeconvolution results of the Mn 2p, Co 2p, and Cr 2p peakXPS spectra are shown in Figures 12A, 12B, and 12C,respectively. The Mn 2p3/2 peaks are centered at ∼640.59-652.01eV, and Mn 2p1/2 peaks are centered at ∼642.20-653.60 eV,with ∆ ≈ 1.6 eV, indicating those of pure Mn3O4 phase.59 TheO 1s peaks (not shown) were located at 529.76-531.76 eV andare assigned to oxygen in Mn3O4. The other shoulder at 533.40eV is attributed to the presence of a hydroxyl species or adsorbedwater on the surface.

For the cobalt oxide NC sample, the two main Co 2p bindingenergies are ∼779.63-794.85 eV (Co 2p3/2), ∼781.70-796.35eV (Co 2p1/2), and ∆ ≈ 2.1 eV, and the shakeup satellites (I)with a low intensity at ∼10.6 eV due to the main Co 2p bandare characteristic of those of pure Co3O4.60 The shakeup satellites(II) at ∼5.6 eV from the weak Co 2p bands suggests thepresence of NC surface hydroxyls. The Cr 2p spectrum of thechromium oxide NC sample shows the Cr 2p3/2 peak at576.88-586.39 eV, and the Cr 2p1/2 peaks at 579.74-588.91eV are attributed to Cr3+.61 The O 1s peak (not shown) is oftenbelieved to be composed of two peaks, related to two differentchemical states of oxygen. The binding energies of eachindividual component are 531.80 (Cr3+-O) and 533.35 eV(OH-). Furthermore, the calculated atomic ratios of Mn:O, Co:O, and Cr:O are found to be very close to 0.73:1.00, 0.85:1.00,and 0.98:1.49, respectively, corresponding to the stoichiometryof Mn3O4-x (x ) 1.25), Co3O4-x (x ) 1.47), and Cr2O3,respectively.

Figure 10. TEM images and corresponding SAED patterns of transitionmetal oxide nanocrystals synthesized using the corresponding metalsalts following Scheme 2: (a,b) 10 nm-sized hexagonal Mn3O4

nanocrystals, (c,d) 3 nm-sized Cr2O3 nanodots, (e,f) 6 nm-sizednanospheres and a few truncated nanospheres of Co3O4, and (g,h) 8nm-sized hexagonal NiO nanocrystals.

Figure 11. TEM images of Co3 times: (a) 12 h, a mixture of nanocubicand nanocubic self-assemblies (sample 12), (b) 18 h, nanocubes (sample11), (c) 24 h, self-assemblied nanospheres nanospheres (sample 10).(d) A suggested scheme for the self-assembly of a spherical shape.

11212 J. Phys. Chem. C, Vol. 113, No. 26, 2009 Nguyen and Do

4. Conclusion

Different two-phase methods have been developed for thesynthesis of two classes of monodisperse metal oxide nano-crystals: rare earth oxides and transition metal oxides, usingmetal salts as starting precursors instead of expensive organo-metallic compounds. The results revealed that for the formationof rare earth oxide nanocrystals, tert-butylamine plays a keyrole as a nucleophile agent. However, in the case of transitionmetal oxide nanocrystals, ethanol is an important agent forreducing the monomeric transition metal precursors to induceNC products. Various sizes and shapes of monodispersenanoparticles such as cubic, spherical, hexagonal, peanut, and

rod NCs were obtained depending on the nature of metalprecursors. The size and shape evolution of CeO2 NCs wasfound to be as a function of monomer concentration of ceriumsalts. The size and shape varied from 3 nm-sized cubes to 8nm × 10 nm-sized elongated cubes, when the cerium monomerconcentration increased from 0.015 to 0.120 M. This could berelated to increasing the chemical potential in bulk solution. Inthe case of Co3O4 nanocrystals, the morphology changed fromnanocube to nanospheres, when the reaction time increased from12 to 24 h at 180 °C, due to the differences in chemical potentialbetween different facets of the nanocrystals.

The SAED patterns of the obtained NC samples show a setof sharp spots characteristic of single crystalline structures, andthe strong diffraction rings indexed accordingly with thestructures determined by the XRD spectra. The XPS resultsrevealed the presence of two oxidation states of cerium,samarium, manganese, and cobalt on the nanocrystal surface.However, it seems that only one oxidation state on the surfacefor Y, Cr, and La oxide nanocrystals is present. Two differentcomponents of the O 1s XPS large peak for these Ce, Sm, Mn,and Co oxide NC samples refer to the defected structure.

These synthetic methods have several advantages, includingrelatively mild conditions, easy manipulation with controllablesize and shape of the NCs, and a possibilty to scale-up to themultigram-scale products using inexpensive metal salt precur-sors. These methods can be extended to the synthesis of otheruniform NCs as well as doped and multicomponent NCs. Furtherwork is in progress in our laboratory.

Acknowledgment. This work was supported by the NaturalSciences and Engineering Research Council of Canada (NSERC)through a strategic grant. The authors thank Profs. S. Kaliaguineand F. Kleitz for stimulating discussions and comments.

Supporting Information Available: XPS spectra of theCeO2 nanocubic sample, TEM images of the as-synthesizedmetal oxide nanocrystals after calcination, TEM images of thealkyl chain-capped metal oxide nanocrystals synthesized asmultigram-scale products, XPS spectra in the wide-energy rangeof the transition metal oxide NC samples, FTIR spectrum ofthe Y-oleate complex precursor, FTIR spectra of alkyl-cappedmetal oxide NCs, TEM image of as-made Y2O3 nanocrystals,N2 adsorption-desorption isotherms and corresponding poresize distribution curves. This material is available free of chargevia the Internet at http://pubs.acs.org.

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JP900226M

11214 J. Phys. Chem. C, Vol. 113, No. 26, 2009 Nguyen and Do

S-1

General Two-Phase Routes to Synthesize Colloidal Metal

Oxide Nanocrystals: Simple Synthesis and

Their Ordered Self-Assembly Structures

Thanh-Dinh Nguyen and Trong-On Do*

Department of Chemical Engineering, Laval University, Quebec G1K 7P4 Canada

To whom correspondence should be addressed. E-mail: Trong-On.Do@gch.ulaval.ca

SUBMITTED TO J. Phys. Chem. C – Jan. 2009

REVISED MANUSCRIPT: APRIL 2009

SUPPORTING INFORMATION

Binding Energy (eV)

0 200 400 600 800 1000

Inte

nsity

(C

PS

)

(a)

(b)

O 1s

C 1s

Ce 3d

Ce 4p

S-Figure 1. Survey XPS spectra of the CeO2 nanocubic sample before (a) and after (b)

calcination.

S-2

Wavenumber (cm-1)

50010001500200025003000

Tra

nsm

ittan

ce (

%)

alkyl -COO-

Y-O

S-Figure 2. FTIR spectrum of Y-oleate complexes.

S-3

Wavenumber (cm-1)

500100015002000250030003500

Tra

nsm

ittan

ce (

%)

(a)

(b)

2928

28491624

1465

724

1090

OH

1555 1460666

S-Figure 3. FTIR spectra of alkyl-capped metal oxide NCs: (a) OM-capped Y2O3

nanorods (sample 7) and (b) OA-capped Co3O4 nanospheres (sample 10).

The FTIR spectra of OM-capped Y2O3 and OA-capped Co3O4 NCs exhibit sharp

peaks at 2828-2949 cm-1 attributed to the C-H stretching mode of methyl and methylene

groups of the alkyl chains (S-Figure 7). The spectrum for OM-capped Y2O3 NCs shows

the characteristic amide II band (1465-1624 cm-1) resulting from the combination of N-H

bending and N-C stretching of –NH2 group of oleylamine.16 For the OA-capped Co3O4

NCs, the broad IR bands at 1555 cm-1 and 1460 cm-1 attributed to the antisymmetric

(COO-) and the symmetric (COO-) stretches of oleic acid.23 The absorption bands at

about 500-750 cm-1 correspond to vibrations of metal oxides. Furthermore, a broad band

at 3250-3500 cm-1 is assigned to O-H vibrations of adsorbed water on the surface. This

clearly indicates oleylamine or oleic acid molecules capped on the nanocrystal surface.

S-4

S-Figure 4. TEM image of as-made Y2O3 nanocrystals synthesized using Y(ac)3

precursors following Scheme 1B.

S-5

Binding Energy (eV)

0 200 400 600 800 1000

Inte

nsity

(C

PS

)

O 1

s

C 1

s

Y 3

d

A

Binding Energy (eV)

152 154 156 158 160

Inte

nsity

(C

PS

)

3d5/23d3/2

B

S-Figure 5. Survey XPS spectrum (A) and Y 3d XPS spectrum (B) of the calcined Y2O3

nanorods (sample 7).

S-6

S-Figure 6. N2 adsorption-desorption isotherms and corresponding pore size distribution

curves (inset) for the calcined rare earth oxide NC samples: (A) Er2O3 (sample 4) and (B)

Gd2O3 (sample 5).

0

10

20

30

40

0.0 0.2 0.4 0.6 0.8 1.0

P/Po

V a

ds (

cc/g

)

0 20 40 60

pore diameter (nm)

dV/d

D

A 28 nm

0

10

20

30

40

50

0.0 0.2 0.4 0.6 0.8 1.0

P/Po

Vads

(cc/

g)

10 30 50 70 90

pore diameter (nm)

B 18 nm

62 nm

S-7

Binding Energy (eV)

0 200 400 600 800 1000

Inte

nsity

(C

PS

)

(b)

(a)

Mn

2p3/

2

Mn

2p1/

2

O 1

s

C 1

s

A

Binding Energy (eV)

0 200 400 600 800 1000

Inte

nsity

(C

PS

)

(b)

(a)

Co

2p3/

2

Co

2p1/

2

O 1

s

C 1

s

B

S-Figure 7. Survey XPS spectra in the wide energy range of the transition-metal oxide

NC samples before and after calcination: (A) as-made (a) and calcined (b) hexagonal

Mn3O4 nanocrystals; (B) as-made (a) and calcined (b) Co3O4 nanospheres.

S-8

S-Figure 8. The color photographes and corresponding TEM images of the alkyl chain-

capped metal oxide nanocrystals synthesized the muti-gram-scale products when the

reaction size for the synthesis was increased by a factor of 12.5:

- (A1) 3.4 g and (A2) 6-8 nm size of OA-capped CeO2 nanocubes;

- (B1) 1.2 g and (B2) 10-20 nm size of OM-capped Er2O3 nanospheres;

- (C1) 2.0 g and (C2) 5-7 nm size of OA-capped Co3O4 nanospheres.

S-9

S-10

S-Figure 9. TEM images of the as-synthesized metal oxide nanocrystals after calcination

in the air at 550oC for 2 h: (a) Sm2O3 (calcined sample 1), (b) CeO2 (calcined sample 2),

(c) Er2O3 (calcined sample 4), (d) Gd2O3 (calcined sample 5), (e) La2O3 (calcined sample

6), (f) Y2O3 (calcined sample 7), (g) Mn3O4 (calcined sample 8), (k) Cr2O3 (calcined

sample 9), (h) Co3O4 (calcined sample 10), and (l) NiO (calcined sample 13).